1. Field of the Invention
This invention resides in the field of electrophoresis performed in slab gels. In particular, this invention relates to slab gel cassettes in which the slab gels are retained in enclosures and often stored in buffer solutions until ready for use.
2. Description of the Prior Art
When a slab gel and the cassette (i.e., the flat-plate enclosure) in which it is cast are immersed in a buffer solution for packaging or storage purposes, the gel tends to absorb water from the buffer solution over time. This uptake of water causes swelling of the gel, and the swelling causes the gel to exert pressure on the plates of the cassette. The pressure causes the plates to bow outward, with the pore size of the gel becoming greater at the center of the gel compared to the edges. When samples are loaded onto the gel and electrophoresis is performed, this variation in pore size causes the solute bands from the sample to migrate faster at their centers, producing an effect on the shapes of the bands known as a “smiling” effect, which increases as the bands migrate greater distances through the gel. This makes reading of the gels difficult and lowers reproducibility and accuracy.
Cassettes that are particularly susceptible to distortion are those in which one or both of the flat plates is made of plastic, and often plastic that is approximately 2.0 mm or less in thickness. While cassettes are available in which one or both plates are made of glass, cassettes in which both plates are plastic offer certain advantages, including ease of manufacture of the plates themselves and ease of sealing the plates to each other along their edges, typically by sonic welding, with appropriate spacers to fix the thickness of the cavity. Electrophoresis “cells” in which the cassettes are placed and that contain electrodes and appropriate electrical connections for imposing an electrical potential along the lengths of the gels are often designed for cassettes of particular sizes which impose an upper limit on the thickness of the cassette walls. Many cassettes are susceptible to distortion for this reason as well.
Another problem that arises in slab gel cassettes is one involving the formation of wells along the upper edge of the gel that are used for the loading of samples onto the gel. These wells are formed by the use of a well-forming insert commonly known as a “comb,” which is a bar with dentiform projections, or “teeth,” along one longitudinal edge of the bar. The comb is placed on the cassette prior to the casting (polymerization) of the gel with the teeth extending into the cavity to form the wells as the polymerization reaction proceeds. Ideally, the thickness of each of the teeth is equal to the gap width between the flat plates of the gel, so that the final gel will extend upwards between adjacent teeth but not into regions between the fronts or backs of individual teeth and the cassette walls where gel material can make it difficult to remove the comb from the cassette and expose the wells. In practical usage, however, avoidance of gel material in these regions is difficult to achieve, and is more so when these regions widen. The comb is also typically made of a material that inhibits polymerization at its surface, thereby preventing the polymerized gel from adhering to the comb. This is of value since adhesion of the gel makes it difficult to remove the comb without damaging the partitions in the gel that separate the wells. The comb material also inhibits polymerization in the spaces between the teeth of the comb and the cassette plates, but tends to lose its ability to do this when these spaces widen or when combs are used that are off-specification or otherwise thinner than expected. Polymerization in these spaces can also occur when too much polymerization catalyst is present or when copper is present in the monomer solution at a high concentration. The formation of gel material in these spaces is known as “webbing,” and interferes with the loading of the samples.
Disclosures of combs designed for use in the casting of electrophoresis gels appear in the following United States patents: Sugimoto et al. (Fuji Photo Film Co., Ltd.) U.S. Pat. No. 4,883,577, Nov. 28, 1989; Bambeck et al. U.S. Pat. No. 4,909,918, Mar. 20, 1990; Chu et al. (Bio-Rad Laboratories, Inc.) U.S. Pat. No. 5,073,246, Dec. 17, 1991; Bettencourt et al. (Bio-Rad Laboratories, Inc.) U.S. Pat. No. 5,164,065, Nov. 17, 1992; Chu et al. (Bio-Rad Laboratories, Inc.) U.S. Pat. No. 5,284,565, Feb. 8, 1994; Steiner et al. (Pharmacia Biotech) U.S. Pat. No. 5,843,295, Dec. 1, 1998; Selby et al. (The Perkin-Elmer Corporation) U.S. Pat. No. 5,993,628 (Nov. 30, 1999); Scott (C. B. S. Scientific Co., Inc.) U.S. Pat. No. 6,139,709, Oct. 31, 2000; Latham (Bio-Rad Laboratories, Inc.) U.S. Pat. No. 7,588,673 B2, Sep. 15, 2009; and Perez et al. (Bio-Rad Laboratories, Inc.) U.S. Pat. No. 7,658,827 B2, Feb. 9, 2010.